The present invention relates to electric watthour meters and, more particularly, to apparatus for mounting a disk and a portion of the gearing of an electric watthour meter.
Conventional electro-mechanical electric watthour meters employ a conductive metal disk rotated as the rotor of a small induction motor by interaction with fluxes generated by opposed voltage and current stators. When the fluxes produced by the current and voltage stators are in quadrature, the rotational torque experienced by the disk is proportional to the voltage applied to the load multiplied by the current consumed by the load; that is, the power consumed by the load. Disk rotation is magnetically resisted in proportion to its rotational speed. Thus, the disk speed is proportional to the power consumed by the load. Each rotation of the disk represents a predetermined increment of energy consumed. The rotations of the disk are accumulated over time in a mechanical or electronic accumulator, or register, for billing purposes by the utility supplying the power.
The induction motor in the electric watthour meter is normally limited to extremely low power dissipation since the power it consumes imposes a parasitic burden on the system. The torque which the watthour meter is capable of developing is also correspondingly low. The low available torque makes it mandatory to minimize the drag imposed on the rotor. Such drag reduction is most critical in the bearings supporting the disk itself and in the gear mesh in the first gearing stage. ln addition, economical manufacture requires a design in which mounting tolerances for the disk and first gearing stage are limited to such small values that the disk and first gearing stage can be installed without requiring adjustment. Furthermore, a design which minimizes the number and complexity of parts aids in reducing cost.
Typically, the disk is supported in the vertical direction by an attractive or repulsive magnetic support bearing, and is located in the radial direction by bearing pins fitted into bearings at the ends of the disk shaft. The first gearing stage usually consists of a worm formed in the disk shaft mating with a worm wheel, or worm gear, mounted on the meter frame. The worm wheel is mounted between bearings in side plates of a bearing cartridge. The bearing cartridge is installed on a machined surface of the meter frame.
The bearings containing the bearing pins at the ends of the disk shaft are mounted in circular holes bored through walls provided in the meter frame for that purpose. The bearing and hole at one end of the frame are frequently of a different diameter from those at the other end of the frame. The holes are bored over-size to permit insertion of the bearings and each is secured in place by a screw which urges the outer surface of the bearing into intimate contact with the inner surface of the hole. Several sources of error accumulate in this technique.
First, it is extremely difficult to maintain close relative positional tolerance in boring a pair of holes in separate walls. Second, the location of the line defining the contact between a cylindrical outer surface of a bearing and a cylindrical inner surface of a hole is indefinite. Third, when bearings and holes of unequal diameter are used, errors in bearing diameter or hole size permit unequal displacement of the axis of the bearings during tightening of the screws and consequently produces an offset between the bearing pins. As a result of these three error sources, the axes of the bearings at the two ends of the frame may be displaced from each other, and one or both may be skewed with respect to the axis of the disk shaft. Such displacement and/or skewing is capable of significantly increasing bearing drag. Finally, it is particularly difficult to maintain absolute tolerances between the holes and other machined surfaces in the meter frame. Thus, the mesh tolerances between the worm and worm wheel, established by the two holes holding the bearings, and the machined surface supporting the worm wheel, are difficult to control.